This invention relates to fluorescent lamp dimming systems and more specifically relates to a novel system to insure seasoning, or burn-in of new (unused) fluorescent lamps before a dimming function can be enabled.
Fluorescent lamp dimming systems are well known. A typical system of this kind is shown and described in U.S. Pat. No. 5,357,170 in the names of Luchaco and Yorgey, issued Oct. 18, 1994 and assigned to the assignee of the present invention and is herein incorporated by reference. Such systems include a dimming ballast which may be mounted nearby to the lamps and which may be conventionally controlled by the output of a conventional programmable lamp controller such as a controller of the type designated as a microWATT controller, a registered trade mark of the assignee of the present invention. The input control to the controller can be derived from any type of device, such as a manually settable dimmer control, an ambient light sensor, an occupancy sensor, a time clock, and security and safety systems, to name a few. The output of the controller to the ballast serves to control the light output of the lamps connected to the dimming ballast.
It is known that some new (previously unused) fluorescent lamps will fail prematurely (in as short as a few days) unless the lamps are burned in or seasoned in a system subjected to dimming. It is also known that fluorescent lamps should be "seasoned" or "burned-in" (these terms are used interchangeably) by operating them for a given length of time, for example, 100 hours, at some given power, usually at full rated current, before the lamps are dimmed. This seasoning operation will condition the lamps and allow them to be dimmed without suffering premature failure after the process is completed. In a more restricted burn-in technique, the lamps are operated without turning off for 100 hours at full rated current.
Many reasons have been offered for the need for this seasoning or burn-in requirement, but it is not yet fully understood, nor is any specific minimum seasoning time or operating power known to insure seasoning of all lamps. However, it is believed that seasoning for about 100 hours at full rated lamp current should season all lamps, although shorter times or new sequences which may use reduced current may be developed and used at some time in the future, but still using the burn-in concepts of this invention as it relates to a dimmer control system.
The technical reasons for the need for burn-in or seasoning are better understood from an analysis of the typical fluorescent lamp. A fluorescent lamp consists of a glass tube which is internally coated with a phosphor; a gas fill typically consisting of mercury and an inert gas such as argon or krypton; and of electrodes which act as the source of, and collection point for, electrons that make up the majority of the "arc" current in the lamp. All three elements of the lamp play an important role in defining the quality of the lamp in operation. All three elements are also subject to lamp-to-lamp and lot-to-lot variations in the manufacturing process.
The manufacturing process begins with the cleaning of the glass and coating it with phosphor. The phosphor is then thermally cured to the glass. The electrode assembly--consisting of filaments attached to support wires and coated with (electron) emissive material (usually barium carbonate), a glass bead for structural integrity, and a glass end cap with an evacuation tube--is then fused to the glass tube. The entire assembly is heated to a high temperature at which the barium carbonate dissociates into barium oxide and carbon dioxide. The carbon dioxide is pumped out of the lamp together with its air fill when the lamp is evacuated. After that, mercury is introduced into the lamp, typically in the form of a drop or pellet, and an inert gas fill is applied to the lamp. Finally, the lamp is sealed from the outside environment, tested, and shipped.
Manufacturing variations exist in all the processes described above. Thus, different amounts of emissive coating are applied to different filaments, there may be lot-to-lot variations in heating profiles and temperatures, as well as fill gas pressures and efficiencies in evacuating the lamp. The end result is that some variation exists between lamps with regard to their impurity content, such as carbon dioxide and water. More importantly, the amount of impurities is typically not measured or monitored in the manufacturing process, so that it is not possible to tell by looking at manufacturing data whether a particular lamp has a high or low quantity of impurities, or of what kind.
The role of the impurities, even in trace quantities, can be detrimental. First, they can cause lamps to exhibit undue flickering or striations when they are dimmed. Furthermore, in extreme cases they can coat the filament and its emissive coating with material, such as carbon, which inhibits the ability of the electrode to function as an electron source for the discharge ("arc"). In this condition, the electrode quickly fails due to excessive ion bombardment from the discharge, and the lamp can fail in a matter of days.
The role of the burn-in is to operate the lamp at some current, preferably at its full rated current for an extended period of time without interruption. This operating mode sets up the "design condition" for the electrode, and develops a proper hot spot in the filament to support the arc current. Past experience has shown that this operating mode is particularly good at "transporting" the impurity materials to the phosphor coating, where they become absorbed in the phosphor structure and never again re-enter the discharge, rather than letting them coat the filament with impurity matter. While this process does not cure the worst possible lamps, it takes care of the majority of problematic impurity issues in most lamps. It is of course possible to find a lamp manufactured in ideal conditions and with an ideal process that does not contain significant impurities, and can be operated, without harm, in dimming conditions straight out of the box but it is not possible at the present time to identify those lamps in a new batch.
For the above reasons, all fluorescent lamps should be "burned-in" at full light output for a period of 100 hours (which is a fairly safe time) before using them in a dimming mode. This will minimize problems with short lamp life.
However, it would be inefficient and unduly expensive to burn-in all fluorescent lamps made by a particular manufacturer since most are intended for use in a non-dimming application and do not require seasoning or burn-in.